Gradient Materials
July 28, 2014
It's unknown who coined the expression, "Change is good," and that's a good thing, since he would be a hated man. No one likes change, and you can be certain that when you hear some
corporate manager say, "Change is good," bad things are about to happen. If there must be change, people would like the change to be gradual. A
decade time scale, preferably a
lifetime, would be nice.
Some things in
nature happen abruptly, and some things happen gradually. The
universe, itself, started abruptly, with the
Big Bang, and the
quantum nature of
matter is inherently abrupt. The
phase transitions of
macroscopic systems can be either gradual (
first order), or abrupt (
second order). An example of a
first order phase transition would be
melting, and an example of a
second order phase transition would be
superconductivity. In superconductivity, you're either
superconducting, or you're not.
Many useful
materials are
layers of one material on another.
Printed circuit boards, in which a
copper layer forms
conductive paths on a
glass-
epoxy or
ceramic substrate, are one example. An extreme example is
integrated circuitry in which layer after layer of
semiconductors and
metals are used to form some useful
electronics. Since different materials have different
mechanical and
thermal properties, bad things can happen at the abrupt
interface between one layer and another.
You can often eliminate the problems of an abrupt interface by using multiple material layers to approximate a gradual change. A good example of this is the
anti-reflection coating on a
glass camera lens. The
refractive index of glass is about 1.5, so there's quite a bit of
reflection of a
light wave in
air (refractive index, 1.0) as it strikes the lens.
Plugging these values into the
reflectance equation,
where
n1 and
n2 are the two refractive indices (in any order), gives you a
reflectance R of about 4% at
normal incidence. Adding an intermediate layer with a refractive index of 1.225 gives 1% at each interface, so the reflectance is halved.
An example of a graded material. A multimode optical fiber with a graded-index core has lower loss than a step-index fiber. (Illustration by Stanisław Skowron, via Wikimedia Commons.)
One major problem with layered materials is the difference in
thermal expansion between layers. This becomes a major problem when the layer is deposited at high
temperature, but used at
room temperature. At one time, I was developing thick
iron garnet layers on
transparent garnet substrates for
magneto-optical sensors. The layers were prepared at about 800
°C by
liquid phase epitaxy, and when they were cooled to room temperature you would often find that the
wafers were bowed; or, worse yet, they would
crack from the mismatch
strain.
Often it's advantageous to have a surface that's highly strained. In a process called
shot peening, or ball peening, an
alloy's surface is pelted by hard metal balls to induce a
compressive surface strain. The strain profile is gradual through the alloy, since there are more impact
forces at the surface than below the surface.
The purpose of such a layer is to enhance
fracture toughness, since the compression tends to close any cracks at the surface before they can grow larger. Many years ago I was a member of a
research team that used compressive layers on articles made from
single crystals to increase their fracture toughness.[1-3]
An international team of
scientists from the
Chinese Academy of Sciences (
Beijing, China),
North Carolina State University (Raleigh, NC), and the
Nanjing University of Science and Technology (Nanjing, China) have recently
published their research on the properties of material gradient structures. Their approach was to
mimic the
microscale-nanoscale change in the
grain-size of some
biological materials to obtain better mechanical properties for
technological materials.[4-6]
As I've often mentioned, a material derives its properties from both its
chemical composition and its
microstructure. Most materials are
polycrystalline; that is, they are composed of a myriad of small single crystals packed together, and the size and orientation of these grains determine many of the material properties, including its
strength and fracture toughness. Smaller grains offer
hardness, and larger grains offer
ductility.
Some natural materials derive their excellent mechanical properties from a
hierarchical structure, mixing smaller and larger structures to give both strength and ductility. I wrote about the hierarchical structure of
spider silk in a
previous article (Spider Silk, March 12, 2012). What the Chinese-North Carolina research team did was to create materials with a gradient in grain size from large grains in the interior to small grains at the surface to derive benefit from their different properties.[6]
Representation of a gradient structured polycrystalline material
(NCSU image by Yuntian Zhu.)
The research team tried this gradient approach on many metals, including copper, iron,
nickel and
stainless steel, and the mechanical properties were improved for them all.[6]
Experiments were also done for
interstitial-free steel. Although this material can be made with 450
MPa tensile strength, it has low ductility, having a strain limit of about 5%. The gradient process produced material with 500 MPa tensile strength, and a 20%
rupture strain.[6]
These improved mechanical properties arise from conversion of an applied
uniaxial stress to
bi-axial stress generated by the gradient. This results in the the accumulation and interaction of
dislocations that leads to additional
work hardening that supplements that of non-gradient materials. The gradient technique is a novel path to superior mechanical properties for an otherwise optimized material.[4-5]
Says
Xiaolei Wu,
professor of
materials science at the
Institute of Mechanics of the Chinese Academy of Sciences and lead
author of the two papers on this research,
"We think this is an exciting new area for materials research because it has a host of applications and it can be easily and inexpensively incorporated into industrial processes."[6]
The research team will now investigate whether this gradient structure approach can be used to engineer
corrosion resistance in existing materials, and also improve their
fatigue and
wear properties. This research was funded by the
U.S. Army Research Office.[6]
References:
- J. Marion, D.M. Gualtieri, and R.C. Morris, "Compressive Epitactic Layers on Single Crystal Components for Improved Mechanical Durability and Strength," J. Appl. Phys., vol. 62, no. 5 (September 1, 1987), pp. 2065-2069.
- Robert C. Morris, Devlin M. Gualtieri, Dave Narasimhan, and Philip J. Whalen, "Epitaxially Strengthened Single Crystal Aluminum Garnet Reinforcement Fibers," U.S. Pat. No. 5,572,725, November 5, 1996
- Devlin M. Gualtieri, Robert C. Morris, Dave Narasimhan, and Philip J. Whalen, "Single Crystal Oxide Turbine Blades," U.S. Pat. No. 5,573,862, November 12, 1996.
- X.L. Wu, P. Jiang, L. Chen, J.F. Zhang, F.P. Yuan, and Y.T. Zhu, "Synergetic Strengthening by Gradient Structure," Materials Research Letters, July 2, 2014, DOI: 10.1080/21663831.2014.935821. This is an open access article with a PDF file available here.
- XiaoLei Wua, Ping Jianga, Liu Chena, Fuping Yuana, and Yuntian T. Zhub, "Extraordinary strain hardening by gradient structure," Proceedings of the National Academy of Sciences, May 5, 2014, DOI: 10.1073/pnas.1324069111. This is an open access article with a PDF file available here.
- Matt Shipman, "Inspired by Nature, Researchers Create Tougher Metal Materials," North Carolina State University Press Release, July 2, 2014.
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